The Hardy Weinberg law is a fundamental principle in population genetics. It maintains that in a large, closed and randomly mating population, the genotype and gene frequencies will remain constant, providing that gene flow, mutation, and selection do not take place (Blood et al., 2007). Although it is unlikely that an actual population in the wild would strictly fit this definition, the Hardy Weinberg principle can nevertheless be used as a simple and useful tool for estimating genotype frequency if we know the allele frequency and vice versa. It can also be used to predict the heterozygosity in a population. Isn't that the same as estimating the expected genotype frequency (the frequency of heterozygotes)
The Equation
How it works:
Figure 1
The equation and the principles behind it can be used extensively in ecological studies for predicting expected phenotypic values and for validating results to exclude error.
We can easily define what is this is not a definition. Better to say, we can easily derive equations to predict the frequency of homozygotes and heterozygotes in terms of the allele frequency, under the assumptions of the Hardy Weinberg principle... homozygous and heterozygous by assigning the alleles the letters p and q. The AAhomozygous allele would be p2 as shown in Figure 1. The Aaheterozygous would be pq, and because there are 2 sets of heterozygous alleles NO. Alleles are not heterozygous, INIDIVUALS are heterozygous at a locus if they get a different allele from each parent. there would be 2pq of the heterozygous in the population. Finally, the aahomozygous would be q2.
The equation used for the Hardy-Weinberg principle is derived from using a simple punnet square NO. This is NOT a Punnet square. A Punnet squire gives the frequency of individuals from a single corss. using alleles A(p) and a(q) (Figure 1). If we assume that these alleles are in equilibrium, as expected from the Hardy-Weinberg principle, then this formula has to be squared. Also if p+q=1, the final formula for all the punnet squares in Figure 1 would be; p2+2pq+q2=1. By simple rearrangement of the equation it would be possible to estimate the proportion of the allele missing. This, however, would only be true if all the alleles were in equilibrium, which is rarely found in nature.
How to use it:
To apply the equation in real world situations the following example procedure can be followed; Figure 2:
AA
Aa
aa
Total;
2500
450
50
3000
Calculate the allele frequencies: p=(AA) + 0.5(Aa) q=(AA) + 0.5(Aa)
p=(2500/3000)+ 0.5(450/3000) = 0.9083 q=(50/3000) + 0.5(450/3000) = 0.0917 = 1.0000 This means that an individual randomly chosen from the above population would be 91% likely to be carrying the A allele, and 9% likely to be carrying the a allele. If the allele frequency for only one of them is known, for example p, then a simple rearrangement of p+q =1 to q=p-1 can be done to find the frequency of the other. Using this principle, allele frequencies can be estimated.
In the case of polyploid individuals, the expected equation would be (p+q)c where c is the level of ploidy. For example; take a tetraploid individual [c=4], the expected equation for the allele frequencies would be (p+q)4 and using binomial expansion the expected frequencies would be; p4, 4p3q, 6p2q2, 4pq3 and q4. Again, we must assume that the species in question is truly polyploid and is in equilibrium.
The Principle
In general, most populations are not under equilibrium in the wild and can be subject to natural selection which will skew allele frequencies and make one type of allele more common than another. For the Hardy Weinberg principle to work, many assumptions have to be made. One of which is that no outside effects, such as predation or environmental effects, will actively effect the population. In other words, we have to assume that the population is separated from other populations and that no individuals are migrating out of it. from generation to generation allele and genotype in a populations gene pool remains the same, this can be said only if Mendelian genetics is talking place. For evolution to occur the Hardy Weinberg equilibrium must be disturbed.
If the 5 conditions are notmet it can be said that evolution will occur and the Hardy-Weinberg equilibrium will no longer exist;
1.Large populations: Large population sizes reduces the chance of genetic drift and therefore alleles remain constant and the fluctuation of the gene pool is not great enough to cause sudden change. Small populations, however, have a small gene pool and so genetic drift can cause an entire allele to disappear changing the gene pool in the next generation. extreme but OK for illustration This can be seen in a population bottleneck where a species crashes down to low numbers with few alleles, and those few alleles can either be lost or not during the bottleneck episode.
2. No Gene Flow: Gene flow causes allele frequencies to change, a population might loose or gain alleles from neighbouring populations. It can also cause two population to become one since the allele frequency is similar to each other with common gene pool. Gene flow can be caused by insets carrying pollen to new populations, one which does not have a certain genotype. so what type of deviation from the HW proportions would that cause? It can also be caused by migration, for example; human migration rate is considerably higher than what is was a hundred years ago which explains the huge diversity in the gene pool of the populations.
3. No Mutations: Mutation during mitosis may not affect the population, but its occurrence during meiosis will cause the gamete to contain alleles that deviate from the normal population. Mutation causes new generations to have a different gene pool, these mutations may have positive effect on the population as it might help it survive should conditions change. a small effect, hardly noticable
4. Random Mating: One of the main assumptions in the Hardy Weinberg principle is that individual choose mates at random giving equal chance to all the alleles. Therefore, if random mating does not take place gametes will not mix but favour one type. this bias can cause allele frequency to change causing selection. again you should say what form of deviation from HW proportions would this cause
5. No Natural Selection: The main concept of evolution is natural selection, it causes population genotypes and phenotypes to change resulting in more suited individuals for that environment. Only the fittest and most adapted will survive and therefore pass on its allele to the next generation. The frequency of disadvantageous alleles will reduce as it is no longer required and may even result in fixation of a more successful allele.
again you should show the different forms of deviation from HW proportions that could arise.
The Hardy Weinberg law is a fundamental principle in population genetics. It maintains that in a large, closed and randomly mating population, the genotype and gene frequencies will remain constant, providing that gene flow, mutation, and selection do not take place (Blood et al., 2007). Although it is unlikely that an actual population in the wild would strictly fit this definition, the Hardy Weinberg principle can nevertheless be used as a simple and useful tool for estimating genotype frequency if we know the allele frequency and vice versa. It can also be used to predict the heterozygosity in a population. Isn't that the same as estimating the expected genotype frequency (the frequency of heterozygotes)
The Equation
How it works:We can easily define what is this is not a definition. Better to say, we can easily derive equations to predict the frequency of homozygotes and heterozygotes in terms of the allele frequency, under the assumptions of the Hardy Weinberg principle... homozygous and heterozygous by assigning the alleles the letters p and q. The AA homozygous allele would be p2 as shown in Figure 1. The Aa heterozygous would be pq, and because there are 2 sets of heterozygous alleles NO. Alleles are not heterozygous, INIDIVUALS are heterozygous at a locus if they get a different allele from each parent. there would be 2pq of the heterozygous in the population. Finally, the aa homozygous would be q2.
The equation used for the Hardy-Weinberg principle is derived from using a simple punnet square NO. This is NOT a Punnet square. A Punnet squire gives the frequency of individuals from a single corss. using alleles A(p) and a(q) (Figure 1). If we assume that these alleles are in equilibrium, as expected from the Hardy-Weinberg principle, then this formula has to be squared. Also if p+q=1, the final formula for all the punnet squares in Figure 1 would be; p2+2pq+q2=1. By simple rearrangement of the equation it would be possible to estimate the proportion of the allele missing. This, however, would only be true if all the alleles were in equilibrium, which is rarely found in nature.
How to use it:
To apply the equation in real world situations the following example procedure can be followed;
Figure 2:
Calculate the allele frequencies:
p=(AA) + 0.5(Aa) q=(AA) + 0.5(Aa)
p=(2500/3000)+ 0.5(450/3000) = 0.9083
q=(50/3000) + 0.5(450/3000) = 0.0917
= 1.0000
This means that an individual randomly chosen from the above population would be 91% likely to be carrying the A allele, and 9% likely to be carrying the a allele. If the allele frequency for only one of them is known, for example p, then a simple rearrangement of p+q =1 to q=p-1 can be done to find the frequency of the other. Using this principle, allele frequencies can be estimated.
In the case of polyploid individuals, the expected equation would be (p+q)c where c is the level of ploidy. For example; take a tetraploid individual [c=4], the expected equation for the allele frequencies would be (p+q)4 and using binomial expansion the expected frequencies would be; p4, 4p3q, 6p2q2, 4pq3 and q4. Again, we must assume that the species in question is truly polyploid and is in equilibrium.
The Principle
In general, most populations are not under equilibrium in the wild and can be subject to natural selection which will skew allele frequencies and make one type of allele more common than another. For the Hardy Weinberg principle to work, many assumptions have to be made. One of which is that no outside effects, such as predation or environmental effects, will actively effect the population. In other words, we have to assume that the population is separated from other populations and that no individuals are migrating out of it. from generation to generation allele and genotype in a populations gene pool remains the same, this can be said only if Mendelian genetics is talking place. For evolution to occur the Hardy Weinberg equilibrium must be disturbed.If the 5 conditions are not met it can be said that evolution will occur and the Hardy-Weinberg equilibrium will no longer exist;
1. Large populations: Large population sizes reduces the chance of genetic drift and therefore alleles remain constant and the fluctuation of the gene pool is not great enough to cause sudden change. Small populations, however, have a small gene pool and so genetic drift can cause an entire allele to disappear changing the gene pool in the next generation. extreme but OK for illustration This can be seen in a population bottleneck where a species crashes down to low numbers with few alleles, and those few alleles can either be lost or not during the bottleneck episode.
2. No Gene Flow: Gene flow causes allele frequencies to change, a population might loose or gain alleles from neighbouring populations. It can also cause two population to become one since the allele frequency is similar to each other with common gene pool. Gene flow can be caused by insets carrying pollen to new populations, one which does not have a certain genotype. so what type of deviation from the HW proportions would that cause? It can also be caused by migration, for example; human migration rate is considerably higher than what is was a hundred years ago which explains the huge diversity in the gene pool of the populations.
3. No Mutations: Mutation during mitosis may not affect the population, but its occurrence during meiosis will cause the gamete to contain alleles that deviate from the normal population. Mutation causes new generations to have a different gene pool, these mutations may have positive effect on the population as it might help it survive should conditions change. a small effect, hardly noticable
4. Random Mating: One of the main assumptions in the Hardy Weinberg principle is that individual choose mates at random giving equal chance to all the alleles. Therefore, if random mating does not take place gametes will not mix but favour one type. this bias can cause allele frequency to change causing selection. again you should say what form of deviation from HW proportions would this cause
5. No Natural Selection: The main concept of evolution is natural selection, it causes population genotypes and phenotypes to change resulting in more suited individuals for that environment. Only the fittest and most adapted will survive and therefore pass on its allele to the next generation. The frequency of disadvantageous alleles will reduce as it is no longer required and may even result in fixation of a more successful allele.
again you should show the different forms of deviation from HW proportions that could arise.
References
Blood, Studdert, & Gay (Ed.), 2007, Saunders Comprehensive Veterinary Dictionary (3rd ed.). Bath: The Bath Press.
Contributing people:
Group name: The Sterile HybridsSecretary to group Joseph Southan ef08072@qmul.ac.uk
Members:
Lalit Mishra - ef08285@qmul.ac.uk ,
Rayhan Ahmed - bt09082@qmul.ac.uk ,
Richard Dittmer - r.g.dittmer@stu10.qmul.ac.uk ,
Yahya Ahmed - ef08213@qmul.ac.uk